Radar

Many radars measure range by transmitting a short pulse of energy at a target and then waiting for the pulse echo to return. The time between pulses then gives the distance, with echoes from close targets returning sooner than echoes from more distant targets. However, short pulses require large peak powers (typically ranging from kW to MW) to produce an average signal strength that will give a clear return over large distances.

These power requirements are impractical for a small portable system, so AVTIS uses a different technique to measure the range to a target: AVTIS continuously transmits a low average power signal of 120mW. By spreading the transmission over time, the total power is then sufficient to get a discernible echo from a target.

Rather than changing the signal strength (as with pulsed radar) range can then be determined in the following way: the signal frequency is ramped with a constant rate of change, i.e. it has a linear slope. As the signal travels to the target and back, the frequency will then change by an amount set by the time taken for the journey: a more distant target returns a larger frequency difference. This change in frequency is measured by simply mixing the received signals with a copy of the transmitted signal together to produce a beat frequency and hence the range.

MMW_FMCWsm

FMCW radar: (a) Frequency Modulated Continuous Wave signal in the time domain (b) Frequency modulation waveform: the time offset delta t between the transmitted (Tx) and received (Rx) signals is set by the distance to reflector (c) The difference or Intermediate Frequency (fIF) is then proportional to range, R